Due to the desirable corrosion resistance and high-temperature oxidation resistance of hot-dip Al—Zn-coated sheet, their application is being extended from the field of construction materials to higher-level fields, such as: home appliances. In humid environment, Al—Zn alloy layer is vulnerable to corrosion and white rust or black spots may be formed on its surface, impairing the surface quality and corrosion resistance of hot-dip Al—Zn-coated sheet. In order to further improve the corrosion resistance of hot-dip Al—Zn-coated sheet, its surface is conventionally passivated.
For example, patent application CN 101332692A discloses a hot-dip Al—Zn-coated chromium-free passivated steel sheet and its preparation method. This steel sheet has a film formed mainly by ammonium fluorotitanate and ammonium meta-vanadate in combination of passivation with phosphorus compound. However, the treating agent provided in this method is totally inorganic, the film is thin and has no improvement on substrate, its self-lubricating ability is poor and the use of vanadium also causes the problem in relation to environmental protection.
For another example, patent application CN 1247777A discloses a method to treat hot-dip Al—Zn-coated sheet with chromium-containing organic resin composition. The resin film formed on the surface of Al—Zn-coated sheet may prevent appearance damage of the Al—Zn-coated sheet during molding, but the existence of chromium will cause environmental pollution.
For another example, patent application CN 1511908A discloses a waterborne resin composition used to treat Al—Zn alloy coated steel sheet. It contains anionic water-dispersible polyurethane, silane coupling agent and water-soluble zirconium compound. The drying temperature is 70˜220° C. Nevertheless, the water-soluble zirconium compound (ammonium zirconium carbonate) used in this method has poor thermal stability and is quickly decomposed at 60° C.; moreover, this method also uses silane coupling agent, which further increases the cost of this method.